The present invention relates to a process and apparatus for assembly of semiconductor modules and, more specifically, to a process and related apparatus for clamping a cover to a substrate of a semiconductor module during a bonding process.
Semiconductor modules, including single chip modules (SCM) and multi-chip modules (MCM), are used in a number of applications. Such modules generally comprise a substrate, a chip mounted on the substrate, and a lid or cover over the chip or chips. The cover is usually attached to the substrate using an adhesive that is heat-cured or a solder that is reflowed.
The lid or cover may have multiple purposes. It may provide mechanical protection of the device from handling and assembly tooling. It may also enhance heat transfer, especially for flip chip packages, where thermal passes are typically used to thermally connect the back side of the chip or chips to the inner or lower surface of the lid or cover.
When the lid or cover is sealed to the substrate, it also provides environmental protection of the devices from chemicals and moisture. When thermal pastes are used to cool flip chips, the seal typically prevents premature drying of the paste. Although some sealed packages must be hermetic, most only need to pass a bubble leak test. Industry competition demands low cost, high volume, and high-yield assembly of such modules.
Stamped fixtures, typically of stainless steel, such as xe2x80x9cAuer Boatsxe2x80x9d manufactured by AUER Precision Company, Inc. of Mesa, Ariz., are prevalent in the industry as fixtures used for such assembly. Referring now to FIG. 1, there is shown the configuration of a typical stamped stainless steel fixture 90 of the prior art for assembly of semiconductor package or module 95. A typical module 95 consists of a substrate 100 and cover 102, the substrate 100 having mounted upon it an integrated circuit chip 104 and having pins 106 extending from the bottom of the substrate 100.
To assemble module 95, substrate 100 with one or more attached chips 104 is set in a baseplate 110 aligned by alignment features or guides 111. An alignment plate 112 is aligned to baseplate 110 using alignment pins 114 attached to the alignment plate 112, each alignment pin 114 comprising a spacer portion 116 and a pin portion 118 adapted to fit in hole 119 in baseplate 110. Substrate 100 and chip 104 are typically prepared with chip underfill (not shown) applied around and wicking under chip 104, thermally conductive paste (not shown) applied on top of chip 104, and seal adhesive (not shown) placed on the surface of substrate 100 where cover 102 will contact the substrate 100. Solder may also be used in place of seal adhesive.
The underfill protects the interface between the chip 104 and substrate 100 and prevents oxidation of the solder balls 103 used to attach the chip 104 to the substrate 100. The thermally conductive paste creates a conductive pathway from the top of the chip 104 to the cover 102, so that heat may be dissipated away from the chip 104 through the cover 102. Finally, the seal adhesive or the solder bonds around the perimeter of the cover 102, sealing the area inside the cover 102 to protect it from oxidation and to prevent paste drying. Cover 102 is then placed on top of substrate 100 so prepared.
Pressure is then applied to press substrate 100 against cover 102, using a clip 120. Clip 120 consists of a bridge 122 having tabs 124 punched through the bridge 122, and prongs 126 attached at both ends of bridge 122. Each prong 126 has an upper stop tab 128, a lower stop tab 130, and an angled end 132. Tabs 124 are spaced to hold the ends of a leaf spring 134 between them. The compression force, usually between 2 to 10 pounds, imparted by the spring 134 xe2x80x9csquishesxe2x80x9d the paste layer on top of the chip 104 to conform it to the space between the chip 104 and the cover 102, has assuring a good conductive connection and cover seal. The force of the spring 134 also seats the cover 102 on the substrate 100, thinning the adhesive, before the cure step.
The clip 120 is inserted manually by squeezing the prongs 126 slightly toward one another and inserting them through alignment plate holes 136 and baseplate holes 138, thus compressing spring 134. Once the lower stop tabs 130 have completely penetrated baseplate holes 138, the prongs 126 are allowed to spring back away from one another, and the lower stop tabs 130 hold the prongs 126 into place to prevent the force of compressed spring 134 from retracting the clip 120. Upper stop tabs 128 prevent the prongs 126 from being inserted too far into baseplate holes 138.
A semiconductor module 95 so assembled is then put in an oven or furnace to heat cure the seal adhesive or to reflow the solder to create a strong bond and seal between cover 102 and substrate 100. A typical stamped stainless steel fixture 90 might accommodate anywhere from one to ten such modules 95, and typically five modules 95 on a single baseplate 110 with a single corresponding alignment plate 112. Other module-assembly fixtures have been developed, however, as detailed further in the description of the invention.
In any such assembly fixture, the force of the spring that compressively holds the cover against the substrate during the adhesive curing or solder reflow step is an important factor in producing an acceptable quality seal between the cover and substrate for modules produced In that fixture. Generally, the higher the spring force, within the force tolerances of the module and fixture components, the better the yield of acceptable quality modules.
Despite the yield advantage of using springs having a higher resistive force to deflection, such springs are more difficult for process operators to use. Special tooling may be required to open and close fixtures using multiple, high-force springs. In addition, certain module designs, such as modules having column grid array (CGA) input/output (I/O) connections, may be easily damaged by using springs having higher resistive forces. Thus, a need exists for fixtures incorporating springs that provide easy manipulation by operators when loading a fixture, but enable high forces during bonding for Improved product yield.
To meet this and other needs, and in view of its purposes, the present invention provides a fixture for assembly of a semiconductor module comprising a substrate and a cover on the substrate. The fixture comprises a baseplate having alignment features, adapted to accept the substrate, and a spring-loading device. The spring-loaded device is mounted over the baseplate and has a shape memory alloy spring engaging the cover.
The shape memory alloy spring may have a lesser force below a transition temperature range, and a higher force above the transition temperature range. The transition temperature range may be above room temperature and below the bonding temperature of a bonding agent, such as solder or an adhesive, that is used to attach the cover to the substrate.
The present invention further comprises a process for assembling a semiconductor module having a substrate and a cover attached with a bonding agent, the process comprising the steps of:
a) loading the semiconductor module into an assembly fixture and aligning a shape memory alloy spring over the module at room temperature;
b) placing the fixture and module into a heating chamber;
c) heating for a designated period of time the fixture and module in the heating chamber at a temperature sufficient to bond the bonding agent and that is above a transition temperature of the shape memory alloy spring so that the spring exerts an elevated force on the module; and
d) cooling the fixture and module to a lower temperature below the transition temperature so that the spring exerts a lesser force on the module, and disengaging the spring at the lower temperature.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.